Solar radiation is made up of two primary components, one of which is a collimated beam of light which is incident upon the surface of the earth from the position of the sun at any given time period. A second component of solar energy is diffuse radiation. This component is available at a surface from various directions and is not collimated. In collecting solar energy, one basic system is the so-called flat plate collector array. In its simplest form, a flat plate collector comprises a sheet of glass or transparent material disposed above an energy absorbing flat plate so constructed that it acts as a black body to absorb heat. At the back of the absorbing surface, a heat exchange fluid, in either gas or liquid form, is circulated to exchange the heat of the sun to the heat exchange fluid, the heated fluid subsequently being used to heat a home, water supply or swimming pool. In a flat plate collector, major energy absorption occurs at the time when the collimated beam of light is overhead or at solar noon. At this time, the maximum energy is incident on the flat plate surface. However, as the collimated beam moves in response to the motion of the sun, particularly during winter in areas away from the equator, the angle at which the collimated beam strikes a horizontal flat surface becomes lower and thereby causes a loss of available energy due to the angle of incidence of the collimated beam. To some extent, this problem is solved by inclining the flat plate collector to compensate for the sun's position.
Another basic solar energy collecting system currently in use is the double-walled tubular glass collector. These collectors are made of glass concentric tubes, one within the other, and sealed together to provide an annular space between them that is evacuated to a hard vacuum, i.e. 10.sup.-4 torr. The outer "cover tube" is transparent and the inner "absorber tube" is selectively coated over the surface that is in the vacuum space. The selective coating is made by vacuum deposition of a thin layer (1000 .ANG.) of aluminum on the inner glass absorber tube's outer surface. Chromium is then electrically vaporized over the aluminum substrate as black chrome to a thickness of about 1500 .ANG.. The selective coating provides an absorbing surface having 0.8 or greater absorptance and 0.1 or lower infrared emittance. The glass concentric tubes are inserted in liquid tight connections in a manifold and placed along opposite sides of the manifold which cross-connects the tubes to each other and serially connects the tubes along the manifold such that a heat exchange fluid is pumped into each of the absorber tubes to fill them and pumped through the tubes in series to extract and collect the heat of the solar radiation absorbed by the absorber tubes. Solar collectors of this type are disclosed in U.S. Pat. Nos. 3,952,724; 4,043,318; 4,018,215; 4,033,327 and the disclosures thereof are hereby incorporated by reference.
The double-walled glass collectors have also been equipped with various reflectors so that diffuse and direct sunlight reach the absorber tubes. See, for example, U.S. Pat. No. 4,002,160. Arrays of tubular glass collectors offer the advantages of operating at a higher output temperature than the flat plate type collector. Additionally, the tubular collectors weigh and cost less than flat plate collectors.
In the past, one measure of the performance of a solar collector has been the ratio of the shortwave solar absorptance (.alpha.) to the longwave thermal emittance (.epsilon.). This ratio, however, does not always adequately define the best performance for a practical solar collector since the ratio of the collecting and emitting surfaces also influence the over-all performance. The performance of a collector further depends on its orientation with respect to the sun. A maximum performance can only be achieved by any solar collector when its surface is orientated perpendicularly to the direct rays of the sun. Thus for solar collectors having a fixed orientation with respect to the sun, maximum performance can only be achieved only when the sun is normal to the plane of the collector surface. This only occurs for a short period of time during the solar year and the .alpha./.epsilon. ratio never reaches a very high value (.alpha./.epsilon.=10) and known collectors based on such principles have never been able to operate Carnot engines with an acceptable efficiency of more than 50%. Since more than 25% of the total energy consumed in this country is used for the heating and cooling of buildings and the provision of hot water, the diversion of this energy demand from fossil fuels to solar energy would result in a substantial reduction in this nation's dependence on foreign energy supplies. Solar energy is a vast and relatively untapped energy source and the need remains for solar collectors of high efficiency and simplicity of structure.